A wide variety of organisms use peptides as part of their host defense mechanism, in vertebrates this supplements the highly specific cell-mediated immune system [Mor, A., Hani, K. and Nicolas, P. (1994) J. Biol. Chem. 269, 31635-31641. Boman, H. G. (1996) Scand. J. Immunol. 43, 475-482]. Antimicrobial peptides have been isolated from species as diverse as bacteria and mammals [Lehrer, R. I., Lichtenstein, A. K. and Ganz, T. (1993) Ann. Rev. Immunol. 11, 105-128]. Generally, these antibiotic peptides have a net positive charge and a propensity to form amphiphilic α-helix or β-sheet structures upon interaction with the outer phospholipid bilayer in bacterial cell membranes [Besalle, R., Gorea, A., Shalit, J., Metger, J. W., Dass, C. Desiderio, D. M. and Fridkin, M. (1993) J. Med. Chem. 36 1203-1209]. In most cases the detailed molecular mechanisms of the antibiotic action are unknown, although some peptides categorised as class L (lytic) peptides are believed to interact with bacterial cell membranes, probably forming ion-channels or pores [Ludtke, S. J., He, K., Heller, W. T., Harroun, T. A., Yang, L. and Huang, H. W. (1996) Biochemistry 35 13723-13728] leading to permeability changes and consequent cell lysis.
Magainins are antibacterial peptides from the skin of the frog Xenopus laeris and are classified as class L antibiotics because they specifically lyse bacteria; other peptides such as mastroparans, a bee venom, lack this specificity as they lyse eukaryotic as well as prokaryotic cells and are called Class L Venoms [Tytler, E. M., Anantharamaiah, G. M., Walker, D. E., Mishra, V. K., Palgunachari, M. N. and Segrest, J. P. (1995) Biochemistry 34 4393-4401]. Anti-biotic resistance exhibited by certain infectious microorganisms is an increasing problem and there is always a need for new antibiotics. Anti-bacterial peptides such as the class L peptides are known and more are being discovered, with the aim of finding a peptide which is highly cytotoxic and preferably specific for prokaryotic cells. There are differences in the structure and composition of lipid bi-layers between eukaryotes and prokaryotes and amongst prokaryotes themselves which mean that different peptides will have widely differing specificities.
As well as magainins and mastroparans, host defense peptides have been isolated from moths and flies (cecropins) and from Horseshoe crab. The direct action of these host defense peptides to repel predators, for example as venoms, is clear. The search for peptides which exhibit antibiotic effects has lead to the identification of other proteins/peptides which would not be expected to have cytotoxic properties. One of these is lactoferrin, an iron transporter which also shows a weak antibacterial effect.
As well as searching for new antimicrobial peptides, more recently it has been sought to enhance the activity of proteins or peptides with known antimicrobial properties. This has been done in the case of bovine lactoferrin by digesting the native protein with gastric pepsin to produce a peptide, lactoferricin B (LFB), which is much more active than the native bovine lactoferrin. LFB is a 25 residue peptide which corresponds to residues 17-41 of bovine lactoferrin. [Bellamy et al. (1992) Biochem. Biophys. Acta. 1121 pp 130 et seq.]. Structure-activity studies have been carried out on magainins and it has been shown, for example, that enhancement of helicity and of the cationic charge leads to higher antibacterial activity [Chen, Y. H., Brown, J. H., Morell, J. L. and Huang, C. M. (1988) FEBS Letters 236, 462-466]. However, such sequence modifications often result in higher hemolytic activity. It is thus an object of the present invention to prepare peptides and/or peptide derivatives which have significant antibacterial activity but preferably have low toxicity, i.e. little effect on normal eukaryotic cells, e.g. low hemolytic activity. While red blood cells may not be typical eukaryotic cells, they provide a convenient way of assaying for toxicity and in any event are a type of cell which should not be lysed to a significant extent by therapeutic bioactive peptides.